What have I done wrong here? (Camera advice)

Looking though todays footage, I noticed that Im getting quite a lot of pixelation or compression in my video. Havent got it in any previous videos (well, a little bit but I know the camera does have limitations)

If you look in the bushy area to the left, it almost looks like a very low bitrate video. Also see the same effect in the reddish bricks. To be fair, it was very overcast today so maybe that played a part?

Camera was set to 4k 30FPS. ISO100, Shutter at 60 (think I may have jumped up to 80 during this little clip) and also using a Taco RC ND16. EV was fluctuating between -0.7 and +.3. D-LOG and AWB were also on.

Anyone care to take a stab at where I where I might have gone wrong? Im no camera guru but I do understand the basics.

Dropbox link below. Its about 200mb as I wanted to upload it and show it as it was recorded. Dropbox - Grain.mp4